Are you asking if you can use fountain pen ink in an Rotring Isograph Technical Pen (for engineering drawing)? You can, but I wouldn't if I was doing any drafting or engineering drawing.

thanks for quick advice, I agree with you using rotring drawing ink, I got two 23ml black & one red as stock , but I wonder if I can use for example Noodlers' black fountain pens ink which is water proof as well and if it will act with the same performance. with your advice I think a small experiment will not harm....as a test not a real drafting ....

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